<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
      *{
        margin: 0;padding: 0;
      }
    .content{
      margin: 100px;
      width: 200px;
      height: 400px;
      background: #333;
      position: relative;
    }
    .content::before{
      content: "";
      display: block;
      width: 0;
      height: 0;
      border: 20px solid transparent;
      border-bottom-color: #333;
      position: absolute;
      z-index: 100;
      top: -40px;
      left: 50%;
      transform:translateX(-50%);
    }
    </style>
</head>
<body>
    <div class="content">
      
    </div>
</body>
</html>